Embodiments of the present invention relate to a Resistive Random Access Memory (ReRAM) device, and more particularly to a ReRAM technology using a Resistive Switch Device (RSD).
Memory devices can be classified into a volatile memory device and a non-volatile memory device. The non-volatile memory device includes a non-volatile memory cell capable of preserving stored data even when not powered. For example, the non-volatile memory device may be implemented as a flash random access memory (flash RAM), a phase change random access memory (PCRAM), a ReRAM, or the like.
The PCRAM includes a memory cell that is implemented using a phase change material such as germanium antimony tellurium (GST), wherein the GST changes between a crystalline phase and an amorphous phase depending on whether heat is applied to the GST, to thereby perform a data storing operation.
A non-volatile memory device (e.g., a magnetic memory, a phase change memory (PCM), or the like) has a data processing speed similar to that of a volatile RAM device. The non-volatile memory device also preserves data even when power is turned off.
ReRAM is a non-volatile memory device and has a thin film whose electric resistance varies depending on an external voltage applied to the thin film. The resistance difference is used in on/off operations.
FIG. 1 is a cross-sectional view illustrating a resistive switch device (RSD) for use in a conventional ReRAM, and FIG. 2 illustrates principles of operations of the RSD.
Referring to FIGS. 1 and 2, an RSD includes a resistive switch 11 between a top electrode 10 and a bottom electrode 12. In this case, the top electrode 10 and the bottom electrode 12 are each formed of metal materials such as platinum (Pt), and the resistive switch 11 may be formed of a resistive insulation layer such as titanium oxide.
The ReRAM having the above-mentioned configuration has been intensively studied since the 1960s. Generally, the ReRAM (MEMRISTOR) has a Metal-Insulator-Metal (MIM) structure that includes a transition metal oxide material. Therefore, if an appropriate electric signal is applied to the ReRAM, memory characteristics of the ReRAM change from one state, in which the ReRAM has a high resistance and is in a non-conductive state (i.e., an OFF state), to another state, in which the ReRAM has a low resistance and is in a conductive state (i.e., an ON state).
ReRAM characteristics are classified as a Current Controlled Negative Differential Resistance or a Voltage Controlled Negative Differential Resistance according to the method used to implement on/off characteristics.
Materials used in a ReRAM that result in ReRAM (MEMRISTOR) characteristics can be classified into the following first to fifth categories of materials, described below.
In the first category, a material having a Colossal Magneto-Resistance (CMR) property or a Pr1-xCaMnO3 (PCMO) material is inserted between electrodes so that it can utilize resistance variation caused by an electric field.
In the second category, a binary oxide material such as Nb2O5, TiO2, NiO, or Al2O3 is fabricated to have a non-stoichiometric composition so that the resultant binary oxide material can be used as a ReRAM material.
In accordance with the third category, a material serves as a chalcogenide material. A current as high as that in a PRAM does not flow in a chalcogenide material. As a result, a phase change is implemented, and an amorphous structure is maintained in such a manner that it is possible to utilize the difference in resistance caused by the difference in threshold voltage of an ovonic switch.
In accordance with the fourth category, chrome (Cr) or niobium (Nb) is doped on a material such as SrTiO3 or SrZrO3 so as to change a current resistance state to another resistance state.
In the fifth and final category, a Programmable Metallization Cell (PMC) having two resistance states is used. In more detail, silver (Ag) having high ion mobility is doped on GeSe by a solid electrolysis process and two resistance states are generated according to the presence or absence of a conductive channel in the medium obtained by an electrochemical reaction, resulting in a PMC.
However, besides these five categories of materials, other materials having memory characteristics acquired by implementation of stable two-resistance states or methods for fabricating such materials have recently been proposed.
FIG. 3 is a graph illustrating a current value that varies with time in a ReRAM utilizing an RSD.
Generally, ReRAM is configured to read data by sensing a current flowing through a resistor. However, the above-mentioned conventional ReRAM must read different resistance values when the ReRAM has a high resistance and when the ReRAM has a low resistance.
When a current flowing through a resistor is sensed, a constant voltage is applied to the ReRAM, and the ReRAM reads a current caused by the constant voltage. Alternatively, if a constant current is applied to a resistor, a voltage flowing through the ReRAM may be measured.
When the ReRAM is in a low resistance state, resistance of the ReRAM can be quickly detected. In contrast, when the ReRAM is in a high resistance state, it takes longer to detect resistance of the ReRAM. That is, when first data is read, a predetermined time is needed until a voltage level of a capacitor becomes constant. The predetermined time corresponds to the T1 section of FIG. 3. Thereafter, a voltage level may be increased or reduced in response to second data. In order to improve a sensing speed, a method for reducing the sensing time using an amplifier has been widely used.
However, if it is necessary for the ReRAM to read a high resistance value using either a dual sensing structure or a sensing operation, the sensing time T1 is increased. That is, when reading first data is read, a predetermined time is needed until a voltage level of a capacitor becomes constant. The predetermined time corresponds to the T1 section. Thereafter, a voltage level may be increased or reduced in response to second data.